In present day air-traffic control operations, the airport radar installation provides for a PPI display in a special darkened room. This is necessary because the PPI presentation is relatively dim and the phosphor of the fairly large display tube is required to maintain a given trace at least until the next succeeding radar return, to provide a history of radar trails. The radar information must be supplied to the brightly illuminated airport tower cab. To accomplish this, the PPI information is applied to a small cathode ray tube having a short persistence phosphor. The display on the tube face is then photographed by a television camera having a long-lag vidicon. The long time delay characteristic of the latter provides the data retention not present in the small cathode ray tube. The television camera output is applied to a very bright television monitor located in the tower cab.
More recently, air-traffic control authorities have required that the computer generated data associated with the radar trails, such as the alpha-numeric tags which identify the aircraft, its flight number, altitude, etc., should also be displayed on the same tower cab television monitor, properly registered with the PPI information with which it is associated. Obviously to be useful, the registration of the data specified to be within a small percentage of the monitor screen diameter, must be rigidly maintained. A method being employed to provide this composite monitor information, involves the use of a second small cathode ray tube, similar to, but isolated from the first mentioned small tube, and the display on this tube of only the computer generated data, e.g. the above-mentioned alpha-numeric tags. The display on this second small tube is then photographed by a second television camera, similar to the first but having a standard (short decay) vidicon in place of the special long-lag vidicon. This short-decay vidicon characteristic provides the non-smearing display of the computer data as it keeps pace with the radar trails. The images from the two television cameras are then combined by appropriate electronic circuits and applied simultaneously to the television monitor. A composite display is thus generated.
A serious problem exists in the system just described, namely, the inability to maintain registration of the radar video and alpha-numerics to within a reasonable specification. This difficulty results because the information displayed is provided by respective substantially independent subsystems. More specifically, one subsystem must account for the PPI deflection system and the analog circuits which interface with it, the characteristics of the small cathode ray tube it uses and the television camera with its long-lag vidicon. The other subsystem also includes a small cathode ray tube in the deflection system associated with the computer generated data, and a television camera with a standard vidicon. In actual practice, the alignment of the former subsystem alone for displaying only radar video has proved to be difficult. The alignment of the combined subsystems to within a specified limit, and the maintenance of this alignment over a period of time and under different service conditions is virtually an impossibility. For example, after an alignment procedure, registration which is within specification immediately, may be well out-of-specification when checked a relatively short time later. Moreover, during operation it is sometimes necessary to change the expansions of the data being displayed, that is, change the size of the display on the screen, which also entails a change in the display centering. Once registration has been accomplished between the two subsystems, it becomes very complicated to maintain it when manipulating settings for gain and offset changes.
What is needed for air-traffic control operations is a system which is uncomplicated, more economical and relatively free of the alignment and registration problems inherent in the above-described equipment. The present invention fills such a need.